The invention pertains to a detector for deriving a digital signal from an input information signal which represents a runlength limited sequence, the runlength having a minimal value m.
The invention further pertains to a reproduction apparatus, for reproducing digital symbols stored on a medium, comprising the detector
The invention further pertains to a receiver, for reproducing digital symbols from a received signal, comprising the detector.
The invention further pertains to a method for deriving a digital signal from an input information signal which represents a runlength limited sequence, the runlength having a minimal value m.
Optical recording methodology relies heavily on the use of runlength limited channel codes. Their purpose is a.o. to match the spectral characteristics of the user information to those of the optical recording channel. RLL codes restrict the number of consecutive like bits in the coded bit-stream, and may be characterized by two parameters, namely (d,k) with (d+1) being the minimum and (k+1) being the maximum allowed number of consecutive equal bits. Examples of such RLL codes include the EFM-code (d=2, k=10) employed in the CD, and the EFM+-code (d=2, k=10) employed in the DVD.
During the read-out of optically-recorded information, the stored bit-stream undergoes several kinds of distortion, a.o. owing to the limited resolution of the laser beam, dynamic defocusing of the read head and disc tilt. In many cases distortion is such that a simple threshold detector (TD) is not adequate to reproduce the distorted data with sufficient accuracy. This can be the case, for example, in DVD, where the recording density is significantly higher than in the CD, resulting in pits and lands of small size and thus in increased inter symbol interference (ISI) from adjacent pits and/or lands. In such situations more advanced detection techniques are necessary to achieve acceptable bit-error rates.
Several techniques have been proposed for use with RLL codes. According to a first class of techniques a bit sequence is estimated from the input signal by means of a maximum likelihood sequence detection, e.g. through the use of a Viterbi algorithm. The Viterbi algorithm is usually tailored to a fixed partial response and the resulting detector is then called a partial response maximum likelihood detector (PRML). It is normally used as a cascade to an equalizer that serves to shape the overall response of the channel and the equalizer to the fixed partial response used. This detector operates in a bit-by-bit fashion, using the received samples and its own estimates for them, in order to recover the most probable transmitted bit pattern. The Viterbi algorithm is described in more detail in xe2x80x9cDigital Baseband Transmission and Recordingxe2x80x9d, by Jan W. M. Bergmans, 1996, chapter 7, pages 301-372.
PRML-detection suffers from an operating speed bottleneck, caused by the bit recursive nature of its critical loop, which comprises at least a comparison and a selection operation. As a result of this speed limitation, application of PRML detection techniques to the high bit-rates of evolving optical recording technologies becomes increasingly problematic.
According to a second class of techniques a correcting stage is included for detecting and correcting runlength violations in a bit-sequence obtained in a first stage. The first stage may, for example, be implemented as a simple threshold detector. The second stage is known as runlength pushback detection (See for example WO 98/27681).
It is a purpose of the invention to provide a detector according to the introductory paragraph which detects a digital signal from an input information signal with an error-rate approximating that of a PRML-detector, while its computational complexity is reduced.
It is a further purpose of the invention to provide a reproduction system comprising such a detector.
It is a further purpose of the invention to provide a receiver comprising such a detector.
It is also a purpose of the invention to provide a method according to the introductory paragraph which detects a digital signal from an input information signal with an error-rate approximating that of a PRML-detector, while its computational complexity is reduced.
According to the invention a detector is provided for detecting a digital signal out of an input information signal which represents a runlength limited sequence, the runlength having a minimal value m. The detector according to the invention comprises
means for generating a preliminary binary signal out of said input information signal,
means for identifying a composed sequence of subsequent bits within the preliminary binary signal which subsequently comprises at least a first neighboring bit of a unipolar sequence of length greater or equal than m+1, one or more further unipolar sequences of length m and at least a second neighboring bit of a unipolar sequence of length greater or equal than m+1, a unipolar sequence being defined as a sequence of bits having the same binary value, and bordering at both sides at a bit having the opposite binary value,
means for generating a set of sequences which can be obtained from said composed sequence by changing polarities of binary values within said composed sequence without violating the runlength constraint, the set comprising the composed sequence obtained from the preliminary binary signal,
means for calculating a path metric for two or more sequences of said set, said path metric being the sum of the branch metrics for the path through the trellis corresponding to said sequence of binary values,
means for identifying the sequence from said set which has the highest likelihood of correspondence to the input sequence represented by the input information signal on the basis of the path metric.
In the detector according to the invention the computation of the likelihood is restricted to unipolar sequences (runs) or sequences of runs having a minimal runlength.
The invention is based on the insight that on the one hand the runs having a minimal runlength are most prone to detection errors as they have the highest frequency. On the other hand the path through the trellis that is based on the preliminary binary signal already has a relatively high likelihood of correspondence to the input sequence represented by the input information signal. Hence it suffices to consider only the set of possible sequences comprising the composed sequence obtained from the preliminary binary signal and those sequences which can be derived from said composed sequence with relatively few variations. Hence the number of calculations for estimating the optimal path is significantly reduced in comparison to the recursive calculations required in a PRML-detector. Hence, in the detector of the invention the computational power is used efficiently in that a relatively high number of errors is detected with a relatively small number of computations.
A preferred embodiment of the invention according to claim 2 is based on the observation by the inventors that the boundaries between subsequent unipolar sequences usually do not shift more than one sample with respect to each other.
A preferred embodiment of the invention according to claim 3 has the advantage that unlikely modifications of the composed sequence are left out of consideration. This further reduces the computational complexity while maintaining a good detection performance.
A further preferred embodiment is claimed in claim 4. It has been observed that often the boundaries between runs in the signal tend to shift in only one direction. In those cases it is therefore sufficient to evaluate only those variations of the composed sequence. This also reduces the computational complexity while maintaining a good detection performance.
The invention is in particular applicable to a reproduction system. In a reproduction system according to the invention for reproducing digital symbols stored on a medium, said reproduction system comprising read out means which include means for projecting a first optical beam along an optical axis at a scanning spot at the medium and for generating an input information signal representing the digital symbols stored on the medium, by measuring an intensity of an optical beam returned from the scanning spot, said reproduction system further comprising movement means for causing a relative movement in a movement direction between the scanning spot and the medium, said reproduction system further comprising a detector according to claim 1, 2, 3 or 4 for deriving a digital signal from said input information signal.
A preferred embodiment of the reproduction system according to the invention is described in claim 6. The tilt measurement enables the signal detector to predict the kind of distortion which occurs in the signal. This can be used to restrict the possible modifications in the composed sequence to those which have the highest likelihood.
The tilt detection means may be implemented in several ways. For example by a separate sensor, which measures a deviation of the position of the beam reflected from the record carrier. Preferably, however, the tilt detection means derive a tilt signal from the signal read from the record carrier. In that way a separate sensor is superfluous. Such tilt detection means are described for example in the earlier filed application WO 00/10165-A1. A practical embodiment of such detection means is claimed in claim 7.
The invention is in particular suitable for application with a channel code which apart from the runlength constraint also has a repeated minimum transition runlength (RMTR). The latter prescribes the maximum allowed number of concatenated unipolar sequences of minimum length. Such a code is described for example in WO 99/63671-A1. If the channel code has a low value for RMTR also the hardware requirements for the detector are modest, and relatively few variations of the composed sequence have to be computed. EFM+ has a RMTR of 15. The channel codes 17PP and EFMCC, both designed for DVR, have a RMTR of 6.
The invention also pertains to a receiver for reproducing a sequence of output symbols from a received signal, the receiver comprising a detector according to the invention which is coupled to a demodulator for demodulating the received signal.
Preferably the detector is coupled to the demodulator via an equalizer.
The invention also pertains to a method for detecting a digital signal out of an input information signal which represents a runlength limited sequence, the runlength having a minimal value m. The method according to the invention comprises the steps of
generating a binary signal out of said input information signal,
identifying a composed sequence of subsequent bits within the binary signal which subsequently comprises at least a first neighboring bit of a unipolar sequence of length greater or equal than m+1, one or more further unipolar sequences of length m and at least a second neighboring bit of a unipolar sequence of length greater or equal than m+1, a unipolar sequence being defined as a sequence of bits having the same binary value, and bordering at both sides at a bit having the opposite binary value,
generating a set of sequences which can be obtained from said composed sequence by changing polarities of binary values within said composed sequence without violating the runlength constraint, the set comprising the composed sequence
calculating a path metric for two or more sequences of said set, said path metric being the sum of the branch metrics for the path through the trellis corresponding to said sequence of binary values,
identifying the sequence from said set which has the highest likelihood of correspondence to the input sequence represented by the input information signal on the basis of the path metric.